For whatever reason I had no idea that Peace Walker was, like, a proper good-lengthed Metal Gear game. Perhaps it was the previous PSP MGS games that threw me or something, compounded by my general unfamiliarity with the series I suppose, but I assumed it was some weird spin-off not-MGS game. I suppose I should probably just buy one of the various HD collections and finally play these for myself.
From what I understand, even Portable Ops has a fair bit of story meat to it – Peace Walker’s initial setup more-or-less assumes you know what happened in Portable Ops (which I didn’t). To my knowledge Ac!d, Ac!d 2, and Portable Ops Plus aren’t canon.
Actually Portable Ops' canon status is complicated: